In the Daily Telegraph, he wrote that "there has been a hollowing-out of those white-collar professions that used to keep the middle class strong."

His definition appeared to be related to jobs and income. But exactly who qualified as middle class was left vague.

Traditional British social divisions of upper, middle and working class can seem out of date. David Cameron - educated at Eton - has described himself and his wife - the daughter of a baronet - as part of the "sharp-elbowed middle classes".

'Hard-working people' is the term modern politicians of all parties prefer

It used to be easier, says Lawrence James, author of the Middle Class: A History. In one of the early attempts at a definition, an Elizabethan writer defined it as "people who live by their wits rather than manual labour", James says. Going even further back, they were the doctor, lawyer and clerk in Chaucer's Canterbury Tales, he says.

But they really came into their own during the industrial revolution and the growth of Empire when "large armies of clerks" were needed, he says.

The Magazine on class

It's often said that the British have a unique obsession with class. Popular culture is riddled with references to it. Foreign visitors struggle to comprehend the complexities of British hierarchy. So what class am I, asks Tom Heyden.

After World War Two, the middle class grew steadily, says David Kynaston, author of Modernity Britain. The faultlines were pretty clear-cut between workers in heavy industry and employees in offices. "A clerk might earn less than a skilled worker. But he would cling on to his middle class status," says Kynaston.

The 1960s were significant in that the UK's service industries caught up with manufacturing. It was the era of Conservative Prime Minister Harold Macmillan, who once asked: "Who are the middle class? And what do they want?"

But it wasn't until the 1980s that the shift really accelerated to services away from heavy industry.

The Thatcher government's sell-off of council housing and share offers for public utilities is often cited as a turning point. Both policies were pitched to workers who aspired to join the middle classes. But Times columnist Matthew Parris, who served in Thatcher's private office before being elected as a Conservative MP, sees it slightly differently.

"In some ways the Thatcher government was following social trends rather than causing them," he says. You cannot examine the middle class without looking at the "death of the English working class", Parris adds. In economic terms - particularly the Marxist sense of the proletariat - the class is no more, he argues.

Now it's widely believed that the old certainties are gone. One of the old-fashioned signs was accent. The children of the middle class once learned to "speak properly" but not any more, says James.

Even the signifiers are harder to spot, says Robert Opie, founder of the Museum of Brands in London. "By the 1990s you get celebrities taking over as an aspiration from the hope of being middle class," he believes.

Today an employee in a low-wage service sector job might not get their hands dirty. But that doesn't necessarily make them middle class.

And it is debatable whether the cliched signifiers of the middle-class lifestyle really are an accurate picture of the middle class.

Median gross annual income for fulltime employees was £27,000 in 2013, according to the Office for National Statistics. For households with two workers the average income is about £40,000. That may not support what are perceived to be the trappings of being middle class in cultural terms - a Volvo or Audi, foreign holidays to places like Tuscany, clothes from Boden, a subscription to the Week and regular glasses of Prosecco.

What we joke about as middle class behaviour is often more accurately associated with the upper middle class, says Dr Jon Lawrence, reader in modern British history at Cambridge University. "A lot of the stereotypes like going skiing are in fact only for that small 10% at most," he says.

There have been attempts to codify the new, more complex class system.

So there isn't just the middle class. There is the lower middle class, the upper middle class and - presumably - a middle middle class.

In April 2013 the BBC teamed up with sociologists and came up with seven new groups. At the top were the "elite". At the bottom were "traditional working class", "emergent service workers" and the "precariat".

In between came the "established middle class", the "technical middle class" and "new affluent workers".

Business and academics have long stratified people by income and profession along the ABCDE scale.

However it's hard to see such technocratic terms catching on in common speech. Perhaps the usefulness of the phrase "middle class" is that it can mean almost whatever you want it to.

Miliband didn't choose the term by accident. Previously he has used the "squeezed middle". To use the label "middle class" and in the pages of the Daily Telegraph suggests he was responding to specific polling advice, Lawrence says.

It's an intriguing move as, by and large, mainstream politicians tend to avoid grappling with the thorny issue of class, he says.

In the US, the term middle class is used to mean what the working class has traditionally meant in the UK, he says. British politicians have in the last decade or so adopted "hard working families" instead. It's code. In both the US and UK it means people who are not on benefits but are not bankers either, Lawrence says.

And yet six out of ten people called themselves working class in 2013's British Social Attitudes survey. The really interesting thing is how resistant to class labelling respondents were.

"Only half of the population spontaneously places themselves as belonging to either class, with others only doing so when prompted to put themselves into one camp or the other," the report explained.

Lawrence thinks it's because of the "pejorative cultural connotations" that go with being middle class - namely, suburban objects of ridicule such as Margot Leadbetter in The Good Life and later Hyacinth Bucket in Keeping Up Appearances.

"It's the secret associations of the middle class going back to the 70s and 80s, that sense of snobbery and social judgement," he says.

Parris says people's self-identification says more about fashion than reality. "The truth is that 60% of the British are not working class," he says.

The term used to mean something distinct - working in heavy industry, men in flat caps, women scrubbing the doorsteps. Now it simply doubles as "ordinary people", he believes.

William Nelson, who wrote a report about class for think tank the Future Foundation, told the Daily Telegraph in 2010 that the best predictor of middle class was accumulating assets for the future such as ISAs and shares.

Guardian columnist Polly Toynbee, who researched income awareness for her book Unjust Rewards, says both rich and poor tend to see themselves nearer the middle than they are. So in income terms, people's perception of what counts as middle class is skewed by their own economic circumstances.

About 85% of people earn under £40,000 a year, Toynbee says. And yet newspapers often talk of an "attack on the middle class" for tax rises on people earning more than this.

In the end it's a fudge, says Lawrence.

"What makes class complicated here is that people elide social status and income," he says. "Essentially we live in a world of Jane Austen but it's just been modernised. And we still obsess about that in a way other countries don't."

Attempting to pin down the middle class might be like trying to drizzle extra virgin olive oil uphill.

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